1. Field of the Invention
The present invention relates to a charge transfer device, and a driving method and a manufacturing method for the charge transfer device and particularly, to a floating gate type-charge detector applied to the output portion of a charge transfer device and a driving method and a manufacturing method for the charge transfer device.
2. Description of the Prior Art
A floating diffusion type-charge detector and a floating gate type-charge detector are generally known as a charge detector which is applied to the output portion of a charge transfer device.
In the case of the floating diffusion type-charge detector, signal charge to be detected is accumulated in a floating diffusion layer provided at the output portion of the detector, and the potential variation of the floating diffusion layer due to the accumulation of the signal charge is amplified by a buffer amplifier which is generally provided in a chip, and then output to the outside.
In the case of the floating gate type-charge detector, the signal charge to be detected is accumulated in a transfer channel below a floating gate provided at the output portion of the detector, and the potential variation which is induced in the floating gate through the coupling capacitance between the transfer channel and floating gate is amplified by a buffer amplifier, and then output to the outside.
In the floating diffusion type-charge detector, the conversion efficiency for conversion of the signal charge to the output voltage can be set to a higher value as compared with the floating gate type-charge detector by reducing the capacitance of the floating diffusion layer. However, it carries out a so-called destructive detection in which signal charge cannot be reproduced once it detects the signal charge, and it has a disadvantage that there occurs a noise which is so-called reset noise.
On the other hand, as compared with the floating diffusion type-charge detector, the floating gate type-charge detector has generally lower conversion efficiency for conversion of the signal charge to the output voltage, however, the signal charge can be non-destructively detected, and at this time occurrence of the reset noise can be prevented.
The floating gate type-charge detector is divided into two types. In one type of floating gate type-charge detector, when a charge transfer element to which this floating gate type-charge device is applied is driven, a bias gate is provided above the floating gate to control the operating point of the floating gate. In the other type of floating gate type-charge detector, a preset transistor for resetting the potential of the floating gate to a reference potential before the charge detection is provided.
FIG. 1 is a schematic diagram showing a conventional floating gate type-charge detector as disclosed in Japanese Laid-open Patent Publication No. 57-27497, and a bias gate is provided above a floating gate. A charge detector shown in FIG. 1 includes terminals 301 and 302 for supplying a driving voltage, transfer electrodes 306, 307, 309 and 310 for a charge transfer element, floating gate 308, output amplifier 304, wire 303 connecting the floating gate 308 and the output amplifier 304 to each other, DC bias gate 315, terminal 314 for applying a DC voltage to the DC bias gate 315, amplifier output terminal 305 of the output amplifier 304, insulating film 311 and semiconductor substrate 312. The transfer electrodes 306 and 309 are connected to the terminal 301, and the transfer electrodes 307 and 310 are connected to the terminal 302.
The charge detector shown in FIG. 1 is operated in a (2+1/2)-phase driving mode by driving pulses .PHI.A and .PHI.B shown FIG. 2. The three electrodes 306, 307 and 308 constitute one row of the charge transfer element. The terminals 301 and 302 are supplied with the pulses .PHI.A and .PHI.B which are shifted in phase by 120.degree., and a suitable DC voltage VC is applied through the terminal 314 to the bias gate 315 so that the offset level of the floating gate 308 is adjusted and set to about a half of the pulse voltage of the pulses .PHI.A and .PHI.B.
The transfer of the signal charge is carried out as follows. That is, when signal charge 313 is transferred and located below the floating gate 308, the voltage which is substantially proportional to the amount of the signal charge 313 is induced at the floating gate 308 through the coupling capacitance between a transfer channel and the floating gate 308, and the voltage thus induced is output as an output voltage through the output amplifier 305 to the outside. At this time, the signal charge 313 is kept below the floating gate 308 without being extinguished, and also it is allowed to be transferred to the adjacent electrode. Therefore, this charge detection method is a so-called non-destructive detection method.
The floating gate type-charge detector is also disclosed in Japanese Laid-open patent Publication No. 5-152558.
With respect to such a floating gate type-charge detector that a bias gate is provided above a floating gate, there is know such a structure that a bias feedback circuit is installed in order to reduce parasitic capacitance of the floating gate and enhance the conversion efficiency for conversion of the signal charge to the output voltage.
FIG. 3 shows a conventional floating gate type-charge detector disclosed in Japanese Laid-open Patent Publication No. 11-040798, and a bias gate and a bias feedback circuit are provided. In the charge detector shown in FIG. 3, N-type buried channel 417 is formed in P-type well 416 on N-type semiconductor substrate 412. Insulating film 411 is formed on the N-type buried channel 417, and transfer electrodes 406B, 406S, 407B, 407S, 409B, 409S, 410B and 410S for the charge transfer element is formed on the insulating film 411. Further, N.sup.- region 418 for forming a potential barrier is provided in the N-type channel 417 below the specific transfer electrodes 406B, 407B, 409B and 410B, thereby implementing a two-phase driving operation. Output gate 419 and floating gate 408 are provided between the transfer gates 407 and 409, and bias gate 415 is provided on insulating film 411 formed on the floating gate 408. The potential variation of the floating gate 408 is passed through buffer amplifier 404 and then output from output terminal 405 to the outside. Further, bias feedback circuit 420 comprising capacitance element 422 and resistance element 423 which are series-connected to each other is provided, and the connection point between the capacitance element 422 and the resistance element 423 is further connected to the bias gate 415.
In the case of the floating gate type-charge detector in which the bias gate is provided above the floating gate as described above, in other words, in the case of the floating gate type-charge detector in which no preset transistor is provided, the electrical operating point of the floating gate is varied in proportion to the charge amount when the floating gate is charged by some cause during the manufacturing process of a charge transfer device to which the above charge detector is applied or at the stage that it is used after the manufacturing.
The bias gate or both of the bias gate and the bias feedback circuit are originally provided to control the operating point of the floating gate. However, when the charge amount is excessively increased, the operating point of the floating gate cannot be sufficiently controlled by only the effect of the capacitance coupling between the bias gate and the floating gate, and consequently the signal charge cannot be detected or the conversion efficiency of converting the signal charge to the output voltage is extremely lowered. Even when the charge amount is not excessively large, it is required to correct the voltage to be applied to the bias gate every individual charge detector if the charge amount is dispersed among a plurality of charge detectors manufactured, and thus the number of parts of a driving device is increased.
In order to avoid the disadvantages described above, some charge preventing countermeasure should be originally taken. However, it is usually difficult to prevent those disadvantages for charge during the manufacturing process, particularly during the wafer diffusion process.